Polydactyly

Polydactyly (presence of extra toes) is the occurrence of more than the usual number of toes and is characterised by a wide phenotypic diversity. So far, two morphological forms of polydactyly have been described: “Mitten Paw” and “Patty Foot”.

In the Mitten Paw (“fist”) form, the additional toes are shorter than the normal ones, and the gap between the extra toes and the other toes is larger than the spacing between the normal toes. In the Patty Foot form, the additional toes are the same length as the normal ones, and the spacing between the extra and normal toes is the same as that between the normal toes (the broad paw resembles a “burger patty”).

Polydactyly may affect a single paw, two, three, or all four limbs; however, the additional toes most commonly develop on the forelimbs or on all four limbs. The number of extra toes also varies.

So far, three variants have been identified that are responsible for polydactyly. These three variants are located in a regulatory sequence of the Sonic Hedgehog (SHH) gene, known as the “zone of polarising activity regulatory sequence (ZRS)”, which is known to be involved in limb development. They were first described in domestic cats in England (UK1 and UK2 variants) and in Hemingway cats in the USA (Hw variant). All three variants are inherited in an autosomal dominant manner, meaning that cats with one or two copies of these variants are very likely to exhibit polydactyly.